1. Field of the Invention
The present invention relates to Fourier transform infrared (FTIR) spectrometers and more particularly relates to a method and apparatus for detecting chemical agents in the atmosphere using an FTIR spectrometer mounted on a movable platform.
2. Description of the Prior Art
FTIR spectrometers are well known in the art. A typical FTIR spectrometer based on a Michelson interferometer is illustrated in FIG. 1. Typically, such FTIR systems have been used in a laboratory setting under controlled conditions to make qualitative measurements based on spectral signature analysis. However, within the last two decades, the FTIR spectrometer has been used to perform quantitative analysis of elements in an open and uncontrolled atmosphere. Such "open-path" applications include industrial monitoring of pollutants from a smoke stack and military monitoring of chemicals used in war zones. However, once the controlled environment of the laboratory is left behind, variables within the measurement path must be neutralized in order to achieve accurate results.
In the FTIR spectrometer of FIG. 1, an infrared source 2 reflecting or emitting from a sample to be analyzed is directed onto a semi-transparent optical beam splitter 4. The beam splitter 4 reflects approximately half (some loss due to losses in the beam splitter) of the infrared signal to a moving mirror 6 and transmits the remaining half of the signal to a fixed mirror 8. The moving mirror 6 is orthogonally aligned to the fixed mirror 8 and the beam splitter 4 is interposed between the mirrors at a 45.degree. angle. The signals reflected off the fixed mirror 8 and the moving mirror 6 are combined by the beam splitter 4 and are reflected onto a detector 10. As the moving mirror 6 travels in a reciprocating fashion on a line parallel to the fixed mirror 8, the pathlength of the signals reflected by the moving mirror 6 varies. This creates a shift in the relative phase angles of the signals being combined by the beam splitter 4. This combination results in both constructive and destructive interference at the detector 10. This interference creates a position versus magnitude signal known as an interferogram. The detector 10 translates the optical interferogram into an analog voltage which is received by an analog to digital (A/D) converter 12. The AID converter 12 creates a digital signal representing the detected optical interferogram signal. The digital signal from the A/D converter 12 is coupled to a computer 14 for digital signal processing to determine the concentration level of chemical species in the atmosphere. A helium-neon (HE--NE) laser 16 is used as a signal source for a secondary interferometer 18 to generate a single frequency sinusoidal time reference. The time reference from the HE--NE laser 16 is received by the A/D converter 12 and functions as a synchronizing clock for the A/D converter 12.
The operation of a traditional FTIR spectrometer is illustrated in the block diagram/flow chart of FIG. 2. This figure begins with an illustration of the previously described interferogram 20. The computer 14 is used to perform a fast Fourier transform (FFT) 22 which translates the time domain interferogram of block 20 into a frequency domain, single-beam spectrum 24. From the single beam spectrum 24, both a background spectrum (baseline spectrum) 26 and analytical spectrum 28 are derived. From the background and analytical spectra, a transmission spectrum 30 is calculated by dividing the analytical spectrum by the background spectrum. Finally, an absorption spectrum 32 is calculated as the negative logarithm of the transmission spectrum.
The background spectrum 26 is required to reduce baseline variations which can contribute to errors in open-path, centerline measurements. The background spectrum 26 is used to convert the subsequent analytical spectra 28 into compensated absorption spectra 32. This eliminates spectral distortions which may result from the characteristics of the source 2, beam splitter 4, detector 10, and interfering components within the measurement atmosphere. Ideally, the background spectrum 26 would be acquired by sampling the target atmosphere at a time when the target gas to be measured is not present. However, in an open-path system, this is not always possible and indirect background spectrum generation techniques are required. One such technique is known as synthetic background spectrum generation. In this method, a background spectrum 26 is created by taking samples of the original spectrum at points where no components are expected, then generating a curve to fit these sample points. A suitable curve fitting function is the polynomial defined by EQU y=ax.sup.2 +bx+c
where a, b, and c are coefficients to be calculated based on a least squares curve fitting algorithm.
3. Description of the Related Art
U.S. patent application Ser. No. 08/743,295, filed on Nov. 4, 1996, entitled "Apparatus and Method for Real-Time Spectral Alignment For Open-Path Fourier Transform Infrared Spectrometers", having Chung-Tao David Wang and Robert Howard Kagann as inventors, and U.S. patent application Ser. No. 08/992,227, filed Dec. 17, 1997, entitled "Apparatus and Method For Real-Time Spectral Alignment For Open-Path Fourier Transform Infrared Spectrometers", having the same inventors and being a continuation-in-part of U.S. patent application Ser. No. 08/743,295, the disclosure of each of which is incorporated herein by reference, disclose methods and apparatus to correct for wave number shifts associated with open-path FTIR spectrometer measurements. FIG. 4 of the drawing of the aforementioned applications (repeated herein as FIG. 3 with the same reference numerals) illustrates a block diagram of an FTIR spectrometer having a topology which is similar to that of the conventional FTIR spectrometer shown in FIG. 1 and including all of the components of the conventional system, but that the computer 14 is illustrated with the elements preferred to implement the digital signal processing algorithms disclosed in the aforementioned applications. These elements include a central processing unit (CPU) 70, which is electrically connected to a random access memory (RAM) 72, electrically alterable read-only memory (EAROM) 74 and read-only memory (RAM) 76. A display 78 is also operatively coupled to the CPU 70 to provide a visual or printed display of the output data. Alternatively, the output data may be ported to another processing unit or computer for further processing or storage.
FIG. 5 of the drawing in each of the aforementioned applications (repeated herein as FIG. 4) illustrates the operation of an FTIR spectrometer which is used to correct for wave number shifts. The FFT (block 22) of the conventional FTIR spectrometer shown in FIG. 1 is replaced with a phase correction and FFT (block 21). In this block, the computer 14 receives the digitally sampled interferogram from the A/D converter 12 and performs a "Forman" phase error correction to this signal. The result is a phase corrected, single beam spectrum as shown in block 24. The phase correction process reduces spectral distortions and errors in concentration measurements due to off-center and, therefore, asymmetrical, interferogram data. The "Forman" phase correction process is discussed in depth in the article "Correction of Asymmetric Interferograms Obtained in Fourier Spectroscopy," by M. L. Forman et al., in the Journal of the Optical Society of America, Vol. 56, No. 1, published in January, 1966, the disclosure of which is incorporated herein by reference.
A real-time frequency alignment step (block 34) is interposed between the steps of generating the analytical and background spectra and calculating the transmission spectrum (block 30). After both the background spectrum (I.sub.o (v)) and analytical spectrum (I(v)) have been shifted in the frequency alignment step, the absorption spectrum can be calculated, as shown in Blocks 30 and 32 (FIG. 4 herein) by implementing the equation: ##EQU1## Once the absorption spectrum has been calculated, classical least squares regression analysis may be employed to calculate the concentration of the elements of interest (block 36). The result is a quantitative analysis output (block 40) suitable for human or machine evaluation.
A passive Fourier transform infrared (FTIR) spectrometer offers a cost-effective solution for remote sensing of industrial pollutants or chemical agent vapors in a battlefield environment. As shown in FIG. 5, the FTIR spectrometer is passive because it detects infrared radiation when there exists a difference between the chemical agent vapor temperature (T.sub.1) and the background temperature (T.sub.2) without using an infrared source. The captured spectral fingerprints are in the form of emission or absorption depending on whether the chemical vapor is warmer or colder than the background. The signal processing algorithm as shown in FIG. 6 carries out three operations:
1. Perform phase error correction, as disclosed in the aforementioned patent applications of the inventor herein;
2. Compute absorption spectrum as the negative logarithm ratio of the sample spectrum over the background spectrum: A(v)=log.sub.10 {I(v)/I.sub.0 (v)}, where the v is the frequency in wave number (cm.sup.-1) units; and
3. Perform classical least squares (CLS) quantitative analysis by minimizing the sum of squared error between the measured absorption spectrum A.sup.m (v) and the reference spectrum A.sup.r (v).
More specifically with respect to the third operation mentioned above, quantitative analysis is based on a multi-component regression model called classical least squares (CLS). Such a technique is described in the article, "Application of New Least Squares Methods for the Infrared Analysis of Multicomponent Samples", authored by David M. Haaland and Robert G. Easterling, published in Applied Spectroscopy, Volume 36, No. 6, 1982, the disclosure of which is incorporated herein by reference. Detection of the presence of a chemical agent is based on comparing the estimated concentration-pathlength product (CL) against three times the standard deviation (3.sigma.) which is defined as the minimum detection limit (MDL). Utilizing a CLS analysis, the FTIR spectrometer can continuously detect chemical agents and monitor the minimum detection limits in various detection regions.
Current FTIR spectrometers for detecting industrial pollutants or chemical agent vapors in a battlefield environment, when the spectrometer is mounted on a movable platform such as a truck or other vehicle, cannot effectively handle spectral variations caused by the constantly changing field-of-view due to the motion of the vehicle. The FTIR spectrometer needs to acquire and continuously update the background spectrum in order to maintain required minimum detection limits (MDL's) and to combat atmospheric interference.
Another problem with conventional FTIR spectrometers is that a high concentration of a pollutant or chemical agent vapor may saturate the extremely sensitive spectrometer, resulting in detection errors.